Classically, the order Crocodilia is divided into
six suborders: Protosuchia, Archaeosuchia,
Mesosuchia, Thalattosuchia, Sebecosuchia, and
the Eusuchia. Today, Crocodilia are familiar inhabitants
of rivers, swamps, and lakes, distributed
in the tropical and subtropical regions of
North and South America, the Caribbean, Africa,
Asia, and Australia. However, crocodilians did not
begin their evolution as amphibious reptiles. The
earliest recognizable crocodilians arose in the Upper
Triassic and appear to have been terrestrial in
nature. Protosuchians are small, averaging below
two meters in length. They have terminal nostrils,
a single row of dermal armor on either side of the
vertebral column, and elongated carpal bones and
hindlimbs. They show other classic morphological
features of crocodilians in that the pubic bone does
not contribute to the formation of the hip socket as
in all other reptiles. Before apparently giving rise
to the Mesosuchia, the first amphibious crocodilians,
the protosuchids, gave rise to another side
group, the Notosuchia. The Mesosuchia appear
to be the ancestors of the living Eusuchia. The
Thalattosuchia are Mesozoic marine crocodilians,
most of which have longirostrine (having long,
thin jaws) mandibles adapted for fish eating.
Some Thalattosuchia, the geosaurs, apparently
were well adapted to the marine environment,
having reevolved a tail fin for aquatic locomotion.
The living Crocodilia are all placed within the
suborder Eusuchia. They are divided into three
families, the Gavailidae, Alligatoridae, and
Crocodylidae, the latter with its two subfamilies,
the Crocodylinae, and the Tomistominae. The
gharials or gavials of Nepal and India comprise
the family Gavialidae. They are longirostrine
crocodilians specialized for fish eating. They attain
large size, growing to nearly nine meters.
Their limbs are reduced more than in most crocodilians
and they are unable to climb over obstacles
of more than a half meter high. The Crocodylidae
are divided into alligatorines, which include the
American and Chinese alligator and the caimans,
and the Crocodylinae, which include members of
the genus Crocodilus, Osteolaemis, and Tomistoma.
The placement of the fourth mandibular tooth in
crocodilians has often been cited as the difference
between these families, in that in alligatorines the
fourth mandibular tooth resides in a socket in the
upper jaw when the mouth is closed. Within the
crocodiles this tooth fits in a groove in the upper
jaw and thus is visible when the jaw is closed. The
longirostrine crocodilians, such as gharials,
tomistomines, and Crocodilus johnstoni, have
longirostrine jaws and have interdigitating teeth.
Thus, the tooth character is equivocal in these
taxa. Another method of dividing the living forms
into the subfamilies has to do with salt tolerance.
With one exception alligatorines are salt intolerant.
Crocodiles are often found in brackish waters
and some, such as Crocodilus porosus, have been
found hundreds of miles off the coast of Australia.
Unlike alligatorines, crocodiles have well-developed
salt glands in the tongue. Gharials,
although not marine, have salt glands in
the tongue as well as an orbital salt gland.
This trait may indicate an oceanic origin for
gharials.

Aquatic Adaptations
Eusuchians are better adapted to the aquatic
environment than their mesosuchian ancestors.
The secondary palate is well developed,
extending the internal nostrils back to
the pharynx. The external nostrils face dorsally
so the crocodiles can stay hidden in the
water. Here a fleshy valve separates off the
mouth from the pharynx and the internal
nostril openings. Eusuchians also have an
earflap or Ohr Klappe that closes off the external
ear from the water during dives. The
Eusuchia have enhanced the dermal armor
and have a biserial or double row of osteoderms
running on either side of the vertebral
column. This dermal armor was once
thought to be evolved for protection but
this system is tied to back muscles that aid
the crocodilians in bending the vertebral
column for locomotion. Dermal armor was
widespread in early archosaurs but is bestdeveloped
in eusuchian crocodilians. All
crocodilians have a strong bite. Although
crocodilians have jaw adductor muscles in
the temporal region of the skull, the largest
jaw adductor originates on the palate. Except for
fish-eating species that have sharp, curved, conical
teeth, most crocodilians have more squatshaped
teeth in the rear of the jaw for crushing.
Crocodilians will often position turtles in the posterior
region of the jaw in order to crush the shells
of those they cannot swallow whole.

Crocodiles as Social Organisms
Crocodilians have complex mating rituals with
both male and female mating displays. Both sexes
engage in bellowing behavior, although male bellowing
is of a deeper tone, as well as head splashing
to announce their presence to potential mates.
Moreover, males will create subaudible vibrations
296 вЂў Crocodiles
Crocodile Facts
Classification:
Kingdom: Animalia
Subkingdom: Bilateria
Phylum: Chordata
Subphylum: Craniata
Class: Reptilia
Subclass: Archosauria
Order: Crocodyla
Suborder: Eusuchia
Families: Alligatoridae (alligators and caimans, four genera,
seven species), Crocodilidae (crocodiles, three
genera, fourteen species), Gavialidae (garials, two
genera, two species)
Geographical location: Every continent except Antarctica
and Europe
Habitat: Variable depending on species; freshwater
lakes, rivers, streams and swamps, estuaries, and
oceans
Gestational period: Approximately three months
Life span: Up to seventy years
Special anatomy: Quadrupedal; biserial dermal armor
consisting of two rows of osteoderms running on either
side of the vertebral column; secondary palate
with fleshy valve; ear flap; verticalized braincase;
elongated carpals; pubic bone is excluded from the
acetabulum; extensive air sinus systems extending
from the Eustachian tubes to the middle-ear cavity
and skull bones
in the water above their backs, vibrating the water
so that it "dances" above the animal's back, as a
sexual display. Males may use bellowing to define
territories during the mating season. Males will
fight over territory and may not tolerate the presence
of other sexually mature males in their territory
during the breeding season. Females may approach
males and by a series of rubbing moves
indicate their intention to mate. Musk glands located
under the lower jaws are used as a sexual
perfume to arouse the male. The male also has
musk glands that are used in response to the female
scent. Mating occurs in the water, with the
male on top of the female. Males have a copulatory
organ, a grooved penis that is inserted into
the cloaca of the female.
Many crocodilians, such as the American alligator
and saltwater crocodile (Crocodylus porosus),
build aboveground nests using foliage, while others,
such as gharials, may dig nests in the sand.
Temperature-dependent sex determination occurs
in crocodilians. Low nest temperatures, below 30
degrees Celsius, produce females, whereas higher
temperatures, above 33 degrees Celsius, produce
males. Intermediate temperatures may produce
males or females depending upon species. Thus,
both sexes are found in a nest, since the temperature
at the top of nestmaydiffer fromthe temperature
at the bottom. While egg numbers vary, many
crocodilians lay forty eggs or more. In populations
studied, only 2 percent of the hatchlings survived
to maturity. When hatching occurs, crocodilians
use an egg tooth to break the eggshell.
Hatchlings inside the nest will bark or grunt to
signal to the mother,whomaydefend the nest and
maybe nearby to come and help dig out the hatchlings.
Female crocodiles may defend the hatchlings
frompredators that include other crocodiles.
In some species, she will carry the hatchlings to
the river in her mouth.

Diet and Locomotion
Diet is dependent upon the stage of life and the
adult size. When crocodiles are juveniles they
may eat insects, arachnids, small frogs, and
snakes. Adults may eat snails, fish, turtles, small
mammals, and birds. The larger species, such as
Crocodylus niloticus, may take large prey such as
wildebeest. Crocodiles may hunt cooperatively in
bringing down large prey that a single individual
cannot handle alone. Others species, such as caimans,
may chase fish into the paths of waiting
members of the population, or may herd fish to
the shore for easy capture. Crocodilians are quite
adept at stealthy behavior. They remain submerged
and will slowly approach prey until they
can leap or lunge to grab the prey in their viselike
jaws. It is not the teeth that kill a prey but rather
the crushing power of the jaws. If the prey is too
large to kill outright, crocodiles will usually
drown the prey animal and then tear it apart by
rolling movements of the body. In this way two or
more crocodiles can cooperate by holding on to
the same prey animal and tearing it apart by combined
rolling actions.
Aquatic locomotion is accomplished by undulations
of the tail. There is a double row of elongated
scutes running down the length of tail that
may aid in swimming. The limbs are normally
held against the body during swimming. Crocodiles
show versatility in their terrestrial locomotion.
Crocodiles are mainly quadrupedal in stance
and gait; the limbs can be held at about 70 degrees
under the body. Some of the smaller species, such
as Crocodylus johnstoni, can gallop, while the saltwater
crocodile, C. porosus, has been observed to
run bipedally. All crocodilians are capable of leaping
to capture prey. They appear to be fond of
belly sliding as a means of locomotion over short
distances, especially in entering the water.

Physiology and Size
Crocodilians are cold-blooded animals and must
thermoregulate by means of evaporative cooling,
using the mouth, and basking and shade usage
behaviors. Crocodilians, like most reptiles, lack
sweat glands. Despite the fact that crocodilians essentially
have a four-chambered heart, there remains
a small opening between the ventricles persisting
as the foramen of Panizza at the base of the
systemic and pulmonary trunks. Nonetheless, the
crocodile heart is too small for the body to maintain
a high metabolic rate. The stroke volume of
the heart and the heart versus body mass ratio determines
whether the animal may maintain high
activity levels. Crocodiles have a metabolic rate
that is about 4 percent that of an adult human.
However, small crocodilians have higher metabolic
rates than do larger species.
Crocodilians come in all sizes. Some of the caimans
are small species, such as Paleosuchus, which
may reach little more than five feet in length. However,
other caimans, such as Melanosuchus, the
black caiman, may reach up to twenty feet. Crocodiles
also vary in length. The dwarf crocodiles of
Africa, Osteolaemis tetraspis, may reach over six
feet in length. The largest crocodilians are crocodiles.
Crocodylus niloticus may reach over twentyone
feet in length, and specimens of Crocodylus
porosus have been known to reach lengths of nearly
thirty feet. Despite their size, some species of crocodilians,
such as the gharials that are over twenty
feet in length, are relatively harmless to humans
because they are specialized for fish catching and
cannot open their mouths very wide.